The Stirling cycle requires two volumes interconnected by a dead space having a regenerator included in the latter. One of the volumes is an expansion space, maintained at a generally high temperature, and the other volume is a compression space maintained at a relatively low temperature. The regenerator is equivalent to a thermodynamic sponge, alternately releasing and absorbing heat when gases are transferred between the two volumes. The dead space consists of that part of the working space not swept by any of the pistons operating within the expansion and compression volumes; this dead space typically will include cylinder clearance spaces, void volumes of the regenerator or heat exchangers, and the internal volume of the associated ducts and ports interconnecting the two volumes.
The amount of flow through the dead space, during each cycle of operation of the Stirling engine is important because flow losses therein affect the net cycle output and the efficiency of the engine. Emperical data has been employed to date to guide the design of the dead space and thereby the regenerator configuration of a Stirling engine.
For example, it has been found that the desirable characteristics for a regenerator matrix should comprise: (a) for maximum heat capacity, a large solid matrix; (b) for maximum heat transfer, a large, finely-divided matrix; (c) for minimum flow losses, a small, highly porous matrix; and (d) for minimum dead space, a small, dense matrix. Clearly, it is impossible to satisfy all of these conflicting requirements. Therefore, with the present state of art for the Stirling cycle, a compromise has been employed; this compromise has resulted in what is known as a fixed regenerator design which will not vary in volume or flow capacity in spite of the fact that the engine itself provides different gas volume flow patterns under different engine loading conditions. Thus, use of a fixed regenerator matrix geometry results in variable flow losses and heat transfer characteristics, dependent on engine operating conditions. Because regenerators are normally sized in order to satisfy some maximum operating condition, part-load efficiency may be improved by modifying the regenerator matrix relative to the full-load requirements.
The difficulty of designing a regenerator system for a Stirling engine is further complicated by the fact that the time for a particle to pass through the regenerator matrix is small compared to the total blow-time; in a Stirling engine, blow times are exceedingly short. For example, at a moderate engine speed of 1200 revolutions/min. or 20 cycles per second, the blow time is 10 times less than the permissible minimum in a gas turbine engine. Since the blow times are so short, it has been demonstrated by other authors that no gas particle passes straight through the regenerator matrix in a single cycle. The actual net flow time through the matrix is about half the complete cycle time, the remaining time being occupied in either filling or emptying, the dead space. As a result, the heat transfer process that occurs is very complex, which involves a repetitive fluid to matrix, matrix to fluid, fluid to matrix cyclic relationship.
It is important therefore that the dead space and regenerator design be improved to permit some adjustment to the changing flow pattern required under different operating conditions.